This present invention is concerned with an improved process for converting the green liquor stream from a kraft paper plant to white liquor.
The kraft process is based on the pulping or digesting of wood chips in a strongly alkaline liquor. This liquor is called white liquor. It is a fluid consisting mainly of NaOH and Na.sub.2 S. The pulping operation consumes the NaOH whereas the Na.sub.2 S content remains substantially constant.
The spent liquor from the digesters plus the filtrate from the washing operation is commonly known as black liquor and contains substantially all the alkali originally added and other wood and wood derived residues from the pulping operation. The black liquor is sent to an alkali recovery plant where it is evaporated and the concentrate then burned. The residue from the combustion process includes Na.sub.2 S and Na.sub.2 CO.sub.3. This residue is dissolved with water to form green liquor. The green liquor will contain, in varying amounts, a high content of sodium carbonate and a minor amount of NaOH. Lime is added to boiling green liquor to form white liquor according to the following reactions:
Slaking reaction EQU CaO+H.sub.2 O.revreaction.Ca(OH).sub.2
Causticizing reaction EQU Ca(OH).sub.2 +Na.sub.2 CO.sub.3 .revreaction.2NaOH+CaCO.sub.3 .dwnarw.
The process does not affect the absolute amount of Na.sub.2 S in the green liquor because Na.sub.2 S does not react in the slaking or causticizing process. The CaCO.sub.3 byproduct mixture formed is known as lime mud or sludge and it is separated off and CaO (lime) is then regenerated in a lime kiln for reuse in the further conversion of green liquor to white liquor.
The concentration or amount of Na.sub.2 CO.sub.3 in the green liquor is the basis for determining how much lime is introduced into the causticizing plant. The heterogeneous nature of the regenerated lime particles and green liquor makes it difficult to precisely control the lime flow so that there will not be variations in causticizing efficiency. These variations also affect the lime mud settling rate in the white liquor clarifier where the white liquor is clarified by separating out the lime mud present.
The settled out mud is removed from the white liquor clarifier for further processing. An online density controller regulates the density (solids concentration) of the lime mud by adjusting the mud flow rate from the clarifier. If the lime mud density is high, the flow from the clarifier is increased to pump out the lime mud until the density of the lime mud is lowered. In the alternative, the density of the lime mud may be raised by decreasing the flow rate.
A torque measurement on the rake mechanism can override this control method. If the torque rises above the target level, the density controller is overridden and additional lime mud is pumped out to bring the torque to a level which will not cause equipment damage and process interruption.
The variation in the lime settling rate and the torque control mechanism used to compensate for such variation can result in widely different flow rates. For example, in a commercial plant these rates have been known to vary from about 150 liters per minute to about 500 liters per minute within 3 to 4 hours.
This cycle may be repeated 6 or 7 times during a day. As a given amount of wash water is introduced counter currently to wash the mud, fluctuations in the mud flow rate would affect the mud washing efficiency, clarity of the wash and the efficiency of the lime kiln operation.
Accordingly, it is a primary object of the invention to provide an improved method for the regeneration of white liquor from green liquor of a kraft paper plant stream.
It is also an object of this invention to provide an improved method of controlling the variations in the operation of the clarifier in the lime mud separation step in the production of white liquor in a kraft paper plant stream by using a controlled rate of lime mud removal.
These and other objects of the invention will become apparent from the text of the specification and the appended claim.